Normal QT Interval Duration
The normal corrected QT (QTc) interval is less than 450 ms in adult men and less than 460 ms in adult women, with values of 390 ms or shorter considered abnormally short in both sexes. 1
Understanding QT Interval Measurement
- The QT interval represents ventricular repolarization time and is measured from the beginning of the QRS complex to the end of the T wave on an ECG 1, 2
- QT interval varies with heart rate and must be corrected (QTc) to provide a standardized value 2
- Lead II is conventionally used for QT interval measurement, as U waves are less prominent in this lead 3
- For accurate measurement, select a lead with T-wave amplitude of at least 2 mm and well-defined T-wave end 2
QT Correction Methods
- Bazett's formula (QTc = QT/√RR) is most commonly used but has significant limitations 1, 2
- Bazett's formula overcorrects at higher heart rates (>80 bpm) and undercorrects at lower heart rates 2, 4
- Linear regression functions are recommended over Bazett's formula for QT-rate correction 1
- Fridericia's formula (QTc = QT/∛RR) is more accurate, especially at higher heart rates 2, 4
- QT correction should not be attempted when RR interval variability is large (e.g., atrial fibrillation) 1
Normal QTc Values by Age and Gender
- Adult men: normal QTc < 450 ms; prolonged QTc ≥ 450 ms 1
- Adult women: normal QTc < 460 ms; prolonged QTc ≥ 460 ms 1
- Children under 12 years: normal QTc < 450 ms 1, 5
- Short QT: ≤ 390 ms in both adult men and women 1
- Gender differences in QTc emerge during adolescence (12-16 years) with an 8 ms difference 1
Clinical Significance of QTc Values
- FDA guidelines recommend reporting three severity levels for QTc: > 350 ms, > 480 ms, and > 500 ms 1
- QTc > 500 ms is associated with increased risk of Torsades de Pointes, a potentially life-threatening ventricular arrhythmia 2, 6
- An increase in QTc > 60 ms from baseline is considered clinically significant 2
- There is no established threshold below which QT prolongation is completely free of proarrhythmic risk 6
Special Considerations
- QT interval is longer in young and middle-aged females than males, with differences of 12-15 ms in younger adults and 6-10 ms in older age groups 1
- Gender differences in QTc become small after age 40 and practically disappear in older adults 1
- QT interval prolongs in ventricular conduction defects and requires adjustment for QRS duration 1
- JT interval (QT duration - QRS duration) can be used when QRS is prolonged 1
- Athletes may have longer QT intervals due to lower resting heart rates, but their QTc values should be within normal limits 1
Common Pitfalls in QT Measurement
- Measurement errors can occur with biphasic or notched T waves; the end of the entire T wave complex should be considered 5
- Normal limits established using percentile distributions are preferable to mean values ± 2SD because QTc distributions are skewed 1
- There is substantial overlap in QTc intervals between normal individuals and those with long QT syndrome, making diagnosis challenging in some cases 7
- Time of day can influence QT interval measurements, affecting serial comparisons 1
- Different recording systems and measurement programs may yield different QT values 1